"Wire bonding machine" is the name given in the semiconductor industry to a class of machines used to complete electrical connections by bonding the ends of lengths of very fine, uninsulated wires to terminal pads on a semiconductor chip to other elements of the circuit of which the chip is a part. In a majority of cases the actual bonding is accomplished by holding the wire with its side held against the surface to which it is to be bonded and then applying sonic energy to the wire. The wire is held in place and the energy is applied to the wire by a "bonding tool". The bonding tool serves the additional functions of positioning the end of the wire over the circuit point at which the end is to be bonded and shaping and directing the wire as it is extended from the bond at a first circuit point to the point at which the second bond is to be made. Moreover, the tool is often employed in the process of severing the wire after the second bond is completed.
To perform these several tasks the working face of the tool may be formed with grooves by which the wire is centered under the face. It may be formed with a sharp edge at the heel or rear of the face to aid in nicking and breaking the wire after completion of the second bond of a wire run. In almost every case it will be formed with a guide hole through which the wire is threaded and by which the direction of the wire is controlled as it is paid out. Because the wire extends through the hole, somewhat like a thread extends through the eye of its needle, wire position and direction can be determined by movement of the tool. However, unlike the eye of the needle, the guide hole of a bonding tool does not extend through the shaft from side to side. It extends instead from an opening at the rear side of the tool to an opening in the bottom face of the tool.
The bonding tool is used in conjunction with a wire clamp. The relation of tool and clamp varies in different machines but in general the clamp may be opened and closed independently of tool movement and it is moved up and down and backward and forward with the bonding tool. In addition it can be moved toward and away from the guide hole on a line of movement which coincides with the axis of the guide hole or at least intersects that axis in the vicinity of the guide hole entrance.
The wire, usually gold or aluminum, may be only one mil or less in diameter and the face of the tool may be only a few mils square. They and the circuit points to which bonds are made are so small that they must be viewed through a stereo microscope. The work and the tool are necessarily viewed from the front which is the side of the tool opposite the clamp. The guide hole entrance opening is at the rear of the tool and the exit opening is at the bottom. Neither opening is visible except with the aid of a mirror.
Threading the bonding wire into the guide hole of the tool is a difficult task which is often long and frustrating. Although bonding machines have been used for many years, threading has always been done by hand because no automatic threading technique or apparatus has been devised previously or, in any event, has not been available commercially.